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This question can tell you a bit more about the frequency filter: https://psspy.org/psse-help-forum/question/5290/how-does-psse-calculate-frequency-at-bus/

The other two relate to extended term simulations with are detailed in the Program Operation Manual 23.10. "Extended Term Simulations" (PSSE 34). The POM has this note regarding extended term simulations:

Note

Although the extended term option exists in models that were developed in the past, newer models added in the last 20 years are not able to run in midterm mode (i.e., these are not midterm ready). To make a model midterm ready, a special intergration mode needs to be developed and this has not been done. Also, vendor supplied user-written models, models not supplied as part of the standard PSS®E library, are not midterm ready. In view of this we have stopped offering the Midterm option for sale to users.

The midterm option was originally created when CPUs ran MUCH slower and CPU time/usage was a much more valuable commodity. Now, with the advent of Ghz CPUs and multiple cores, the need for this specially coded performance is no longer as great as the normal state-space simulation can still be run for long duration simulations.

The midterm activities MSTR and MRUN are still available for use, but please note that your network will most likely contain a mixture of midterm and non-midterm models. This mixture of models will NOT yield a consistent solution. For that reason, we are discouraging the use of the MSTR and MRUN activities.